This invention relates generally to oil field equipment and more particularly but not by way of limitation to a pump jack for reciprocating an oil well pump.
Heretofore a pump jack of the type having a walking beam rotatably mounted on an "A" frame sampson post has been the standard in the oil industry. At one end of the walking beam is mounted a sector head or commonly called a "horse head." The sector head includes a cable line attached to a polish rod. The polish rod is attached to connected pump rods and a pump inside the well. A polish rod load is the combined weight of the polish rod, pump rods, pump, and the oil being pump to the ground surface. One end of a vertically disposed connecting rod is attached to the other end of the walking beam. The other end of the connecting rod is connected to rotating counterweights. The counterweights are rotated by a speed reduction box driven by a drive motor. Because of the pump jack's inherent design the combined weight of the walking beam, connecting rod, counterweights, and the polish rod load are placed on the sampson post. This combined weight requires the pump jack to be constructed of heavy metal castings including a reinforced base on a suitable foundation in order to carry this heavy load. Also because of the weight of the pump jack additional horsepower requirements are placed on the drive motor for raising and lowering the polish rod load.
In operation the drive motor drives the gear box which rotates the counterweights. The connecting rod attached to the counterweights move the walking beam up and down. As the walking beam moves up and down, the sector head raises and lowers the polish rod, pump rods, and pump. At the peak of the sector head's upstroke or downstroke sufficient time must be given for the string of pump rods to completely move upward or downward. This time period is called lag time and is measured in degrees. Under normal pumping conditions and when the torque load requirements for lifting the polish rod load have been balanced, the counterweights will rotate approximately 175.degree. moving the sector head to the peak of its upstroke. At this point there is an average 5.degree. lag time for the complete string of pump rods to move upward before the counterweights which are continuing to rotate start the sector head on a downstroke. If the 5.degree. lag time is not sufficient time for the entire string of pump rods to complete the upstroke, the downstroke of the sector head will start the string of rods moving downward and the pump rods will snap. Pump rod snap will cause rod shock and excessive wear on the rods and pump. Insufficient lag time causing rod snap is common in deep wells where the combined weight of the string of drill rods will cause the rods to stretch as the rods and pump are raised and lowered.
There have been various departures from the above described pump jack having a walking beam. None of the prior art pump jacks disclose the advantages and unique features of the improved jack as herein described.